Much study has been devoted to blood coagulation, platelet aggregation and fibrinolysis in respect to their contribution to the pathogenesis of thrombosis and thrombolysis. Local stasis and lesions of the vascular wall are also important contributory factors in thrombosis and thrombolysis. It is generally accepted that the adherence of platelets to each other and to the vascular wall is the first step of white thrombus formation. Blood coagulation is important for the stability and further growth of platelet thrombi. The exact mechanisms of in vivo thrombin and fibrin formation, in relation to the initial platelet aggregate, and to whether or not tissue factors are released, are insufficiently understood. The endogenous or spontaneous activation of the fibrinolytic system also needs further clarification. The participation of activated Hageman factor in thrombolysis is an important phenomenon and the elucidation of its exact role in fibrinolysis will offer a better understanding of the in vivo mechanisms for the resolution of a thrombus. These mechanisms and related topics, including other aspects of hemostasis, thrombosis, and the fate of formed thrombi, have long been and will continue to be our broad field of research interest. Specifically our immediate research objectives are to study (1) the role of Hageman factor in thrombolysis, and (2) whether or not Hageman factor participates in the release from tissues of plasminogen activator.